Electronic and optical modeling of solar cell compound CuXY<Subscript>2</Subscript> (X = In,Ga, Al; Y = S,Se, Te): first-principles study via Tran–Blaha-modified Becke–Johnson exchange potential approach

The electronic and optical properties of the ternary CuXY₂ (X = In, Ga, Al; Y = S, Se, Te) compounds have been investigated using the density functional theory based on the full potential linear augmented plane wave method via Tran–Blaha-modified Becke–Johnson exchange potential techniques. From this study, it is found that these compounds are direct band gap semiconductor and in good agreement with reported results. The optical properties such as dielectric tensor components and the absorption coefficient of these materials are determined in order to investigate their usefulness as solar cell materials.     

Optoelectronic and thermoelectric properties of KAuX5 (X = S, Se): a first principles study

The electronic structure as well as optical and thermoelectric properties of the orthorhombic polychalcogenides of gold KAuX₅ (X = S, Se) compounds have been investigated using full-potential linearized augmented plane wave within the framework of the density functional theory (DFT). The local density approximation (LDA), generalized gradient approximation (GGA) by Perdew, Burke and Ernzerhof (PBE), Engel–Vosko generalized gradient approximation (EV-GGA), and the recently modified Becke–Johnson approximation (mBJ) formalism are used for the exchange correlation energy to calculate the total energy. The results show that KAuX₅ (X = S, Se) is a direct band gap semiconductor at Γ–Γ point. The total and partial density of states indicate that the states Au-d, S-p, and Se-p of both compounds have strong contributions to valence band in the energy range from ?10 up to 0.0 eV. One can notice from electronic charge density that both compounds show greater iconicity and smaller covalency. Optical properties with photon incident energy up to 14.0 eV have been calculated and analyzed. Important transport properties such as Seebeck coefficients as well as thermal and electrical conductivities and effective mass are obtained and discussed in details.     

First principles investigation of electronic, optical and transport properties of α- and β-phase of arsenic telluride

Crystalline arsenic telluride exists in two stable phases. The monoclinic α-phase transforms to rhombohedral β-phase under high pressure. The electronic, optical and transport properties of the two phases has been investigated using full potential linear augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of DFT with generalized gradient approximation (GGA). We present the energy bands, density of states and optical properties like the complex dielectric functions and absorption coefficients. From the dynamic dielectric constant, the structural anisotropy for the monoclinic α-phase is clearly observed, whereas the longitudinal and transverse components are almost identical for the β-phase. The optical absorption profiles clearly indicate that β-phase has possibility of greater multiple direct and indirect interband transitions in the infrared and visible regions compared to the α-phase. The rhomohedral phase which has the Bi₂Te₃ type structure has the possibility of thermoelectric properties, therefore transport properties like electrical and thermal conductivities, Seebeck and Hall coefficients etc. are also calculated. Good agreements are found with the available experimental results.     

First principles calculations of structural,electronic, optical and thermodynamic properties of PbS,SrS and their ternary alloys Pb1−xSrxS

Using first-principle method, we investigate the structural, electronic, optical and thermodynamic properties of the strontium semiconductors Pb_1?xSr_xS with 25%, 50% and 75% of Sr. The calculations are performed by using the full-potential linearized augmented plane wave (FP-LAPW) method. As exchange–correlation potential we used the generalized gradient approximation (GGA) of Perdew et al. The variation of the calculated equilibrium lattice constant versus concentration shows that a small deviation from Vegard’s law is clearly visible with downward bowing parameter equal to 0.009 Å. The bulk modulus as a function of x for Pb_1?xSr_xS alloy shows a significant deviation from the linear concentration dependence (LCD) with downward bowing equal to 6.62 GPa. The different roles of structural and chemical effects on the gap bowing and its variation with composition are identified and discussed. In addition, density of states and optical properties were calculated and compared to the available experimental data and previous theoretical works. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.     

The role of f-states in the electronic and optical properties of rare-earth trifluorides (RF3, R = Ce and Gd): a full potential study

A theoretical investigation of the role of f-states on electronic and optical properties of CeF₃ and GdF₃ is presented. Our calculations are based on density functional theory using the full potential linearized augmented plane wave method with the inclusion of spin–orbit coupling. We employed the Coulomb-corrected local spin density approximation (LSDA + U), known for treating the highly correlated 4f electrons properly. The LSDA + U calculations shift the f-states away from Fermi energy (E _F) yielding an insulating ground in agreement with experiment. We find that electronic structure is greatly influenced by the occupancy of f electron. Our calculated reflectivity spectra are compared with the experimental data. The value of calculated reflectivity for both CeF₃ and GdF₃ compounds stays low till ~7 eV which is consistent with their large energy gaps. The results are analyzed in the light of band to band transitions. We have also presented the frequency-dependent dielectric constant and corresponding spectrum of the refractive index for these compounds.     

Structural and optical properties of sulfurized Cu2ZnSnS4 thin films from Cu–Zn–Sn alloy precursors

This study reports the preparation of Cu₂ZnSnS₄ (CZTS) thin films by magnetron sputtering deposition with a Cu–Zn–Sn ternary alloy target and sequential sulfurization. The effects of substrate temperatures on the structural, morphological, compositional as well as optical and electrical properties were characterized. The results showed the CZTS thin films prepared by sulfurization at substrate temperature of 570 °C yielded secondary phases along with CZTS compound. The relatively good properties of CZTS thin film were obtained after sulfurization at substrate temperature of 550 °C. This CZTS film showed compact structure with large grain size of 900 nm, direct optical band gap of 1.47 eV, optical absorption coefficient over 10⁴ cm^?1, resistivity of 4.05 Ω cm, carrier concentration of 8.22 × 10¹⁸ cm^?3, and mobility of 43.38 cm² V^?1 S^?1.     

Synthesis of Cu2ZnSnS4 films from sequentially electrodeposited Cu–Sn–Zn precursors and their structural and optical properties

The Cu₂ZnSnS₄ (CZTS) films are successfully prepared using a process of sequentially electrodeposited Cu–Sn–Zn precursors by a novel electrolyte formula and optimized parameters on Mo substrate, succeeded by annealing in saturated sulfur atmosphere. The results show that the Cu/Sn/Zn precursor sequence is strict, and optimized electro-deposition parameters are as follows: ?0.6 V, 5 min for Cu, ?1.2 V, 2 min for Sn, and ?1.35 V, 10 min for Zn. Layered precursors firstly alloy into Cu₆Sn₅ and CuZn binary phases under low annealing temperature. Then Cu₆Sn₅ and CuZn alloys decompose in sulfur atmosphere, and form CuS, SnS and ZnS binary phases. Cu₂SnS₃ ternary phase forms through reaction between CuS and SnS with increasing the temperature. Finally, the CZTS film is synthesized through reaction among binary and ternary sulfides. The photoluminescence peak from the CZTS films synthesized at 550 °C for 1 h is about at 1.49 eV.     

Thermal evolution of morphological,optical, and photocatalytic properties of Au–Cu2O–CuO nanocomposite thin film

Plasmonic nanocomposite thin films find exciting applications in environmental remediation and photovoltaics. We report on thermal annealing driven development of morphology, structure and photocatalytic performance of Au–Cu₂O–CuO nanocomposite thin film. Nanocomposite thin film coatings of Au–Cu₂O–CuO, prepared by radio frequency (RF) magnetron co-sputtering, were annealed at different temperatures. Thermal annealing driven evolution of morphology of Au–Cu₂O–CuO nanocomposite was studied by field emission scanning electron microscopy (FESEM), which revealed significant growth in size of nanostructures from 10 nm to 69 nm upon annealing. X-ray diffraction (XRD) together with Raman studies confirmed the nanocomposite nature of Au–Cu₂O–CuO film. UV-visible diffuse reflectance spectroscopy (UV-vis-DRS) studies showed band gap variation from 2.44 eV to 1.8 eV upon annealing at 250 °C. Nanocomposite thin film annealed at 250 °C exhibited superior photocatalytic activity for organic pollutants [methylene blue (MB) and methyl orange (MO)] decomposition. The origins of thermal transformation of morphological, optical and photocatalytic behaviour of the Au–Cu₂O–CuO nanocomposite coating are discussed.

     

Effect of sulfur and copper amounts in sol–gel precursor solution on the growth, crystal properties, and optical properties of Cu2ZnSnS4 films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sol–gel spin coating using precursor solutions prepared with copper acetate, zinc acetate, tin chloride, and thiourea in methanol and ethylenediamine followed by sulfurization. Sol–gel precursor solutions were prepared with different amounts of sulfur and copper, and their effects on film growth, crystal properties, and optical properties of CZTS films were investigated. CZTS film thickness increased with the amount of metal salt in the precursor solution. This is attributed to an increase in solution viscosity and a decrease in the solution density/viscosity ratio. All CZTS thin films exhibited kesterite structures with absorption coefficients larger than 10⁴ cm^?1 in the visible region. Band gap energy increased with increasing amounts of sulfur and decreasing amounts of copper. The blue shift of the band gap is attributed to changes in the degree of p–d hybridization related to Cu d- and S p-levels. The role of sulfur and copper on Hall mobility and carrier concentration was investigated. By optimizing the metal salt ratio in the precursor, CZTS film with a resistivity of 5.3 × 10^?2 Ωcm were prepared.     

Structural and optical properties of Cu–N codoped ZnO thin films deposited by magnetron cosputtering

ZnO films codoped by copper and nitrogen were prepared by magnetron cosputtering. The effects of this codoping on the structural and optical properties of ZnO films were systematically studied. The results show that Cu–N codoping didn’t affect the optimal orientation. Cu–N codoping can enlarge the grain size, enhance the crystallinity, reduce the stress and lead to denser and smoother surface. A significant red shift of absorption edge and gap-narrowing effect resulting from codoping were found in ZnO films. Cu-doping, N-doping, and oxygen vacancy are the main factors leading to property modification of the ZnO films. By Cu–N codoping, we can modulate the microstructure and optical properties of ZnO films in a wider range.     

Application of Taguchi approach to optimize the sol–gel process of the quaternary Cu2ZnSnS4 with good optical properties

Present research deals with the optimal deposition parameters configuration for the synthesis of Cu₂ZnSnS₄ (CZTS) thin films using the sol–gel method associated to spin coating on ordinary glass substrates without sulfurization. The Taguchi design with a L9 (3⁴) orthogonal array, a signal-to-noise (S/N) ratio and an analysis of variance (ANOVA) are used to optimize the performance characteristic (optical band gap) of CZTS thin films. Four deposition parameters called factors namely the annealing temperature, the annealing time, the ratios Cu/(Zn + Sn) and Zn/Sn were chosen. To conduct the tests using the Taguchi method, three levels were chosen for each factor. The effects of the deposition parameters on structural and optical properties are studied. The determination of the most significant factors of the deposition process on optical properties of as-prepared films is also done. The results showed that the significant parameters are Zn/Sn ratio and the annealing temperature by applying the Taguchi method.     

CuInGaSe2 crystals synthesis by selenization of Cu–In–Ga alloy in H2Se atmosphere for solar cell applications

Copper indium gallium diselenide (CuInGaSe₂) crystals were synthesized using two step growth strategy. A facile solution route was employed as a primary step to synthesize Cu–In–Ga (CIG) metallic precursor using ethylenediamine as a solvent. Thin films of CIG metallic precursor have been deposited using spray deposition technique on to molybdenum coated soda lime glass substrate under inert atmosphere. The subsequent step involved the selenization of metallic precursor thin films in H₂Se atmosphere at 450 °C for 90 min followed by annealing in Ar thus yielding solar cell applicable dense CuInGaSe₂ crystals. The surface morphology, phase structure and composition of the deposited films were analyzed by field emission scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy and electrical resistivity measurement respectively. The results revealed that annealed films were crystalline in nature exhibiting homogeneous single chalcopyrite phase.     

Ab-initio calculations of synergistic chromium–nitrogen codoping effects on the electronic and optical properties of anatase TiO2

Electronic and optical properties of compensated and noncompensated (Cr, N) codoped TiO₂ have been investigated using density functional theory with plane wave basis set and pseudopotential. To investigate the formation of defect pair in the codoped models, defect pair binding energy was calculated. Compensated codoped model has two Cr atoms doped at Ti sites, one N atom at O sites along with an oxygen vacancy that gave stable configuration, better electronic and optical properties. Defect pair binding energy of this model showed that, individual defects would bind each other leading to stable configuration compared to mono-doped models. Band structure results showed that compensated (Cr, N) codoping introduced substantially broaden intermediate states in the forbidden band along with narrowed band gap. Furthermore, the Fermi level was shifted from top of the valence band to middle of the forbidden band describing half metallic character. Cr doping changed the nature of N 2p states from unoccupied to occupied which will improve electron–hole pair separation. Optical properties comparison showed that all doped models effectively shifted the absorption edge of TiO₂ towards visible light. Compensated (Cr, N) codoped TiO₂ has better optical properties and covered wide absorption band in the visible light region, attributed to the stable configuration, narrowed band gap and widely distributed states in the band gap. Our results provide reasonable explanation of the experimental findings.     

Characterization of Ge–Bi–S glass by thermal, electrical, switching and optical measurements

A study of the synthesized Ge22.5Bi7S70.5 glassy system has been carried out. Differential thermal analysis data indicate the retention in the as-quenched sample of two amorphous phases. Thermal conductivity, , measurements on bulk sample reveal that the main contribution to is due to phonon thermal conductivity. Thermal evaporation of the synthesized ingot gives films with Ge20.7Bi6.8S72.5 as composition. The values of the activation energy and the pre-exponential factor calculated from the direct current electrical conductivity above 53 °C suggest that carrier conduction occurred between extended states in these films. The I–V characteristics in the off-state and the switching phenomenon are investigated. A memory switch with a threshold voltage decreasing with temperature is detected for the studied films. Optical parameters such as absorption coefficient, optical gap and refractive index are also determined. Comparison with binary Ge–S glass reveals that the addition of Bi introduces additional absorbing states at band edges. © 1998 Kluwer Academic Publishers     

Structural,magnetic, optical,and impedance properties of SrxCo1−xFe2O4 nanoparticles prepared by sol–gel method

Sr²⁺ ions-substituted cobalt ferrite having the composition formula Sr_xCo_1?xFe₂O₄ (x?=?0.05, 0.10, 0.15, and 0.20) was synthesized with sol–gel method. X-ray diffraction (XRD) patterns proved that the samples had a spinel structure. The Fourier-transform infrared (FTIR) spectra showed that the samples have two characteristic bands about the intrinsic vibrations of spinel ferrite. The morphology and chemical elements were tested by scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). The elemental state of annealed samples was examined by X-ray photoelectron spectroscopy (XPS). The magnetic data showed the coercivity increased from 954.33 Oe to 1545.54 Oe when the Sr²⁺ ions content increased. The optical absorption properties of annealed samples were investigated by UV–Vis spectroscopy. Complex impedance spectroscopy showed that grain resistance of Sr_0.10Co_0.90Fe₂O₄ sample was 443.9 Ω.